Two thin lenses of focal lengths `f_(1)` and `f_(2)` are in contact. The focal length of this combination is

Options

A. `f_(1)+f_(2)`
B. `(1)/(f_(1)+f_(2))`
C. `(f_(1)f_(2))/(f_(1)+f_(2))`
D. `(f_(1)+f_(2))/(f_(1)f_(2))`

Answer

C

Explanation

Focal length of combination of two thin lenses,
`(1)/(f)=(1)/(f_(1))+(1)/(f_(2))=(f_(1)+f_(2))/(f_(1)f_(2)) Rightarrow f=(f_(1)f_(2))/(f_(1)+f_(2))`